This adds the compiled binary version of all variants, directly from the
upstream zip file.

This adds the "bootloader_only_binaries" directory from commit df68980
(Updated 'Congratulations' sketch in the combined bootloader...) from
https://github.com/rogerclarkmelbourne/STM32duino-bootloader.

This intentionally does *not* use the binaries that use the welcome
sketch, since that sketch disables the debug port (since it is built
with Roger Clark's Arduino_STM32 core, which disables the debug port
unconditionally currently).

This reverts commit 84e4091.

Temporarily reverted, because the tool package change needed for this
does not seem to be published yet.

This allows uploading the HID bootloader and the stm32duino bootloader
(aka maple bootloader 2.0) to the bluepill and blackpill boards (other
boards could also be added, but a bootloader version must be explicitly
selected for each board).

The bootloader is chosen based on the "Upload method" selected and
uploaded with the selected programmer using openocd.

This uses the non-fastboot bootloader, since the current bootloader
autoreset for this bootloader just does a normal reset. If that would be
modified to set the right RTC register with a magic value to stop the
fastboot in the bootloader (pretty much exactly like the hid
bootloader), we could instead use the fastboot version.